Zinc-electroplated steel sheets have good appearance and high corrosion resistance, and therefore they are widely used for various purposes. Usually the corrosion resistance of the plated layer is improved by formation of chemical conversion coatings.
The formation of chemical conversion coatings has been effected with chromate type or phosphate type chemical conversion solutions. Attempts have been made to improve these chemical conversion solutions by modifying the compositions thereof. However, it is difficult to obtain dense and uniform coatings excellent in corrosion resistance by chemical conversion. Chemical conversion coatings are susceptible to abrasion and scratching regardless of how they are improved, and plated layers are often exposed as the result thereof. Therefore, improvement in corrosion resistance by means of chemical conversion has its limit.
Recently there have been into use methods for improving the corrosion resistance of the plated zinc layer per se by forming a zinc layer containing an ingredient or ingredients other than zinc which enhance corrosion resistance. A typical example is Zn-Ni-alloy electroplating.
The Zn-Ni-alloy-electroplated layers which have been developed so far contain as high as 8-16% by weight of expensive nickel and therefore, the cost of the plated layer is about twice that of the conventional Zn-electroplated layer of the same coating weight. In order that Zn-Ni-alloy-electroplated layers can exhibit their intrinsic corrosion resistance, that is, to achieve corrosion resistance 4-6 times as high as that of the conventional zinc-electroplated layers, it is necessary to provide a thick plating as no less than 30 g/m.sup.2 or more (on one side), which is very thick as electroplating. If only stable corrosion resistance is wanted, the coating weight of at least 20 g/m.sup.2 per side is required, and it does not follow that the coating weight can be reduced simply because less corrosion resistance is required. Therefore, the cost of plating is remarkably higher than with the conventional zinc electroplating. The reason why formation of plated layers as thick as 30 g/m.sup.2 (one side) is required in order to realize the intrinsic corrosion resistance of the layer is that no dense corrosion product which exhibits high corrosion resistance is formed on the surface of the plated layer until the plated layer is considerably corroded. If the coating weight is less than 30 g/m.sup.2 (one side), the plated layer remaining as a metal after the corrosion product has been formed is too thin, and thus it cannot maintain corrosion resistance for a long period of time because of loss of the corrosion product. Also, if the coating weight is less than 30 g/m.sup.2 (one side), generation of pin holes is remarkable and corrosive liquid reaches the substrate steel through the pin holes and develops corrosion at the interface between the steel substrate and the plated layer resulting in peeling of the plated layer. This phenomenon is called "corrosion peeling" of the plated layer and is a serious defect of conventional Zn-Ni-alloy-electroplated steel sheets. Steel sheets electroplated with a Zn-Ni alloy at the coating weight of 20 g/m.sup.2 (one side) develop fatal corrosion peeling to such an extent that the plated layer easily peels off in an adhesive tape peel-off test after being exposed to an atmosphere of 70.degree. C. and 98% relative humidity for 96 hours.
Although corrosion peeling can be considerably prevented by chromate treatment after plating, this if far from a complete solution. Further, once plated sheets are chromate-treated, phosphate treatment can no longer be properly effected, which means that the plated sheets can be used only for limited applications.
It is known that Zn-Ni-alloy-plated layers which contain a small amount of titanium have better corrosion resistance (Japanese Laying Open Patent Publication No. 104194/83). Titanium is added to the plating bath in the form of titanium salt (sodium or potassium titanium fluoride, otherwise called sodium or potassium hexafluorotitanate, for example). But it is difficult to cause a significant amount of titanium to codeposit in a Zn-Ni-alloy electroplated layer.
The reason is surmised to be that titanium exists in the plating bath in the form of negative complex ions and therefore does not easily undergo hydrolysis.
We tried to develop Zn-Ni-alloy-electroplated steel sheets which exhibit very stable high corrosion resistance with a coating weight of less than 30 g/m.sup.2 and we found that the corrosion resistance of plated layers can be remarkably improved by causing cobalt and aluminum or magnesium in addition to titanium to codeposit in the plated layer. We also found that corrosion resistance of the above-mentioned plated layer can be further improved by heating the electroplated steel sheets in an atmosphere of 60.degree.-200.degree. C.
On the basis of these findings, we further searched for a balanced composition which enhances corrosion resistance and prevents generation of pin holes, and we succeeded in completing this invention.